Enable -mergefunc-use-aliases

If the Rust LLVM fork is used, enable the -mergefunc-use-aliases
flag, which will create aliases for merged functions, rather than
inserting a call from one to the other.

A number of codegen tests needed to be adjusted, because functions
that previously fell below the thunk limit are now being merged.
Merging is prevented either using -C no-prepopulate-passes, or by
making the functions non-identical.

I expect that this is going to break something, somewhere, because
it isn't able to deal with aliases properly, but we won't find out
until we try :)

This fixes #52651.

src/librustc_codegen_llvm/llvm/ffi.rs

@@ -1358,6 +1358,7 @@ extern "C" {
     pub fn LLVMRustDebugMetadataVersion() -> u32;
     pub fn LLVMRustVersionMajor() -> u32;
     pub fn LLVMRustVersionMinor() -> u32;
+    pub fn LLVMRustIsRustLLVM() -> bool;
 
     pub fn LLVMRustAddModuleFlag(M: &Module, name: *const c_char, value: u32);
 

src/librustc_codegen_llvm/llvm_util.rs

@@ -70,6 +70,9 @@ unsafe fn configure_llvm(sess: &Session) {
         if sess.opts.debugging_opts.disable_instrumentation_preinliner {
             add("-disable-preinline");
         }
+        if llvm::LLVMRustIsRustLLVM() {
+            add("-mergefunc-use-aliases");
+        }
 
         for arg in &sess.opts.cg.llvm_args {
             add(&(*arg));